Conductive adhesives can be used to form electrical interconnection between two conductive materials. Many applications exist for such adhesives including electrical grounding for handheld devices, electromagnetic shielding, and die attach adhesives. For many such applications a conductive paste or a conductive film might be used, depending on specific performance and application details.
However, in any conductive joint, whether formed by a conductive adhesive or other material, electrical reliability is a concern. Stable electrical performance should be exhibited over extended periods even in the face of exposure to harsh conditions, which can be replicated in a controlled setting through accelerated conditioning using bias humidity testing, hygrothermal aging, and thermal cycling. A reliable electrical interconnect should exhibit little or no change in resistance over time under harsh conditions.
One example of an application that depends upon high electrical reliability and low electrical joint resistance is a charge collection tape for use within a photovoltaic (e.g., solar) cell. Such tapes can be used to capture a charge being generated within the cell and then to carry it to external electrical circuitry. Examples of regions within a photovoltaic cell where charge collection tapes might be used include the charge collection grid, ribbon connections between cells, and busbars connecting positive and negative charge collection electrodes to external circuitry. The benefits of conductive tape are a generally well-defined bondline, efficient and rapid adhesive tape application (often in a roll to roll/web process), and cost efficiency.
High reliability of electrical interconnects within a solar cell is important to maintaining the long expected functional lifetime of such a device. Most solar panels are rated to perform for 20-30 years at high efficiencies. As such, highly reliable/robust electrical interconnects and conductive adhesives are likely to be important in maintaining performance targets across long lifetimes.
One class of conductive adhesive used in electrical interconnects is conductive pressure sensitive tapes. A typical tape utilizes a metal foil backing coated with an electrically conductive adhesive composition. A typical conductive adhesive composition comprises a pressure sensitive adhesive (PSA) resin matrix filled with conductive particles that either individually bridge the bondline or form a percolation network that bridges the bondline. Such metal-backed conductive PSA tapes are typically referred to as “charge collection tapes.”
Resins used for conductive PSAs are soft materials with a shear modulus below about 3×105 Pa when measured at a rate of 1 radian/second, which is related to their ability to serve as a pressure sensitive composition. As such, PSA resins are viscoelastic systems that exhibit a certain degree of molecular level flow/viscous character. The viscous character of conventional non-reactive PSAs can affect joint resistance and joint reliability when used in conductive PSA tapes if the tape becomes sufficiently stressed. In particular, high temperatures further soften known conductive PSAs, increasing their tendency to flow.
Other known solutions are also unsatisfactory for a variety of reasons. Pastes do not provide adequate strength for handling until they are thermally cured. Heat seals require either heat tacking or full thermal curing with pressure in order to develop significant bond strength.
There is a need for more electrically reliable materials for applications which are subjected to extreme environmental conditions. Thus, there remains a need for materials that exhibit high reliability and high conductivity (i.e., low joint resistance) under harsh conditions and high temperatures.